Historical Rules · 1 October 2009
Rule 116.6
Official source text from the 1 October 2009 Rules release
Some mana costs contain no mana symbols. This represents an unpayable cost. An ability can also have an unpayable cost if its cost is based on the mana cost of a spell with no mana cost. Attempting to cast a spell or activate an ability that has an unpayable cost is a legal action. However, attempting to pay an unpayable cost is an illegal action.